Ships are designed so that when loaded, they can be sank enough to secure an appropriate draft and stabilized, with their propeller screw also under water. Accordingly when not loaded, ships are floated up by buoyancy too much to be stabilized or to secure a submersing depth of the propeller screw. Unloaded cargo ships and the like therefore take in the liquid to be treated at the port of call and store the liquid to be treated in the hull so as to have a height of the waterline close to that in a loaded state. The liquid to be treated taken in as above is called ballast water.
Ballast water is transported to the next port of call as a “weight” of the ship, and released when cargoes are loaded. In other words, marine organisms in the previous port of call can be brought to the next port of call. Transplanting organisms from one place to another place is likely to result in destruction or contamination of ecosystems grown by Mother Nature in the latter place. Therefore the International Maritime Organization (IMO) has adopted the Ballast Water Convention (International Convention for the Control and Management of Ships' Ballast Water and Sediments) which defines standards for an amount of organisms included in ballast water to be discharged.
According to the standards, the number of organisms of 50□m or greater (mostly zooplankton) included in 1 m3 of ballast water discharged from a ship must be less than 10. The number of organisms of greater than or equal to 10 □m and smaller than 50 □m (mostly phytoplankton) in 1 mL must be less than 10. The number of cholera bacilli in 10 mL must be less than 1 cfu. The number of Escherichia coli in 100 mL must be less than 250 cfu. The number of Enterococci in 100 mL must be less than 100 cfu. Here, “cfu” stands for colony forming unit.
To meet the standards, a lot of techniques related to ballast water treatment have recently been disclosed. Specific examples include a method for removing aquatic organisms by filtration, centrifugal separation, etc., a method for physically or mechanically killing aquatic organisms, a method for thermally killing aquatic organisms, and a method for injecting chemicals into the ballast tank or generating chlorine-based substance and the like to kill aquatic organisms.
The method for removing aquatic organisms by centrifugal separation is free of problems of filter clogging and the like, effective at separating aquatic organisms having certain specific gravities, and is used in combination with other methods. FIG. 8 shows a water treatment device 100 disclosed in Patent Literature 1.
This water treatment device 100 includes a seawater intake line 101, a rough filtration device 102 which is provided with a hydro-cyclone for removing large objects from the seawater taken in, a pump 103 which serves as a ballast water supply device for conveying the seawater, a disinfectant supply device 104 which supplies a disinfectant for killing filtered microorganism and bacteria, a retention tank 105 which retains the disinfectant-added seawater for a predetermined time, a treated water conveyance line 106 which conveys the treated water led out of the retention tank 105, and a ballast tank 107 which stores the treated water conveyed from the treated water conveyance line 106.
Patent Literature 1 does not disclose a detailed configuration of the rough filtration device (hereinafter, referred to as a “centrifugal solid-liquid separation device”) 102. However, the configuration of FIG. 7 has been known for a centrifugal solid-liquid separation device.
Referring to FIG. 7, a centrifugal solid-liquid separation device 200 includes a plurality of cyclones 202 and 204, lower liquid containers 206 and 208 which are arranged under the cyclones 202 and 204, valves 210 and 212 which are arranged under the lower liquid containers 206 and 208, and a storage container 214 which is provided under the valves 210 and 212. A flushing pump 216 is arranged in an upstream side of the storage container 214. The storage container 214 is connected to a discharge port 220 of the hull via a drain pipe 218 in the downstream side of the storage container 214.
The cyclones 202 and 204 include liquid inlets 202a and 204a, liquid outlets 202b and 204b, and lower liquid ports 202c and 204c. A water intake pump 232 and an inactivation device 230 are arranged in an upstream side of the centrifugal solid-liquid separation device 200. A treated liquid storage tank 240 is arranged in a downstream side of the centrifugal solid-liquid separation device 200.
An operation of the cyclones 202 and 204 will be described by using the cyclone 202 as an example. When operating the cyclone 202, the valve 210 is closed to fill the lower liquid container 206 with a liquid to be treated. The liquid to be treated taken into the hull passes through the inactivation device 230 and flows into the cyclone 202 through the liquid inlet 202a. The cyclone 202 has an inverted circular truncated cone shape, and its inner surface 202i has a downward inclined surface. The liquid to be treated flowing into the cyclone 202 through the liquid inlet 202a swirls along the inner surface 202i of the cyclone 202.
During swirling, aquatic organisms having high specific gravities move down along the inclined surface of the inner surface 202i and fall through the lower liquid port 202c into the lower liquid container 206. Meanwhile, water components of the liquid to be treated are taken out of the liquid outlet 202b and conveyed to the treated liquid storage tank 240. When a certain amount of aquatic organisms are accumulated in the lower liquid container 206, the aquatic organisms and the liquid to be treated are discharged into the storage container 214 via the valve 210 below.
The storage container 214 is a pipe having a rather large capacity, and can store a certain amount of aquatic organisms as well as trash, organic substances, inorganic substances and other minerals and hydroxides (such as calcium hydroxide and magnesium hydroxide) (hereinafter, referred to as “aquatic organisms and the like”).
The flushing pump 216 is used to flush water through the storage container 214 when the amount of the liquid to be treated in the storage container 214 reaches or exceeds a certain amount, or at regular time intervals. The aquatic organisms and the like are discharged out of the hull via the drainpipe 218 and the drain port 220, when the valves 210 and 212 are closed so as not to allow the aquatic organisms and the like in the storage container 214 to flow back.